# 0.5 to 0.6
This release introduces a slightly obscure, but
backwards-incompatible, change.
In the generic APIs of versions 0.4 and 0.5, fields whose names began
with a "`_`" character would have this character removed. This no
longer occurs, as [it was both buggy and
surprising](https://github.com/bos/aeson/issues/53).
Also:
* Fixed [a bug in generic decoding of nullary constructors](https://github.com/bos/aeson/issues/62)
# 0.4 to 0.5
When used with the UTF-8 encoding performance improvements introduced
in version 0.11.1.12 of the text package, this release improves
aeson's JSON encoding performance by 33% relative to aeson 0.4.
As part of achieving this improvement, an API change was necessary.
The `fromValue` function in the `Data.Aeson.Encode` module now uses
the text package's `Builder` type instead of the blaze-builder
package's `Builder` type.
# 0.3 to 0.4
## Ease of use
The new [`decode`
function](http://hackage.haskell.org/packages/archive/aeson/latest/doc/html/Data-Aeson.html#v:decode)
complements the longstanding `encode` function, and makes the API
simpler.
[New examples](https://github.com/bos/aeson/tree/master/examples) make
it easier to learn to use the package.
## Generics support
aeson's support for data-type generic programming makes it possible to
use JSON encodings of most data types without writing any boilerplate
instances.
Thanks to Bas Van Dijk, aeson now supports the two major schemes for
doing datatype-generic programming:
* the modern mechanism, [built into GHC
itself](http://www.haskell.org/ghc/docs/latest/html/users_guide/generic-programming.html)
* the older mechanism, based on SYB (aka "scrap your boilerplate")
The modern GHC-based generic mechanism is fast and terse: in fact, its
performance is generally comparable in performance to hand-written and
TH-derived `ToJSON` and `FromJSON` instances. To see how to use GHC
generics, refer to
[`examples/Generic.hs`](https://github.com/bos/aeson/blob/master/examples/Generic.hs).
The SYB-based generics support lives in
[Data.Aeson.Generic](http://hackage.haskell.org/packages/archive/aeson/latest/doc/html/Data-Aeson-Generic.html),
and is provided mainly for users of GHC older than 7.2. SYB is far
slower (by about 10x) than the more modern generic mechanism. To see
how to use SYB generics, refer to
[`examples/GenericSYB.hs`](https://github.com/bos/aeson/blob/master/examples/GenericSYB.hs).
## Improved performance
* We switched the intermediate representation of JSON objects from
`Data.Map` to
[`Data.HashMap`](http://hackage.haskell.org/package/unordered-containers),
which has improved type conversion performance.
* Instances of `ToJSON` and `FromJSON` for tuples are between 45% and
70% faster than in 0.3.
## Evaluation control
This version of aeson makes explicit the decoupling between
*identifying* an element of a JSON document and *converting* it to
Haskell. See the
[`Data.Aeson.Parser`](http://hackage.haskell.org/packages/archive/aeson/latest/doc/html/Data-Aeson-Parser.html)
documentation for details.
The normal aeson `decode` function performs identification strictly,
but defers conversion until needed. This can result in improved
performance (e.g. if the results of some conversions are never
needed), but at a cost in increased memory consumption.
The new `decode'` function performs identification and conversion
immediately. This incurs an up-front cost in CPU cycles, but reduces
reduce memory consumption.